Build layer coverage analysis

ABSTRACT

Examples include apparatuses, processes, and methods for generating three-dimensional objects. An example apparatus comprises build material distributor and a controller. Examples distribute a particular build layer in a build area of the apparatus over a previous build layer. Examples determine a build layer temperature corresponding to the particular build layer. Examples analyze build layer coverage for the particular build layer based at least in part on a build layer temperature of the previous build layer and the build layer temperature of the particular build layer. Examples selectively recoat the particular build layer with additional build material based at least in part on the build layer coverage for the particular build layer.

BACKGROUND

Additive manufacturing systems that generate three-dimensional objectson a layer-by-layer basis are convenient way for producingthree-dimensional objects. Examples of additive manufacturing systemsinclude three-dimensional printing systems. The quality of objectsproduced by additive manufacturing systems may vary widely based on thetype of additive manufacturing technology used.

DRAWINGS

FIG. 1 provides a block diagram of some components of an exampleapparatus for generating three-dimensional objects.

FIG. 2 provides a block diagrams of some components of an exampleapparatus for generating three-dimensional objects.

FIG. 3 provides a block diagram of some components of an exampleapparatus for generating three-dimensional objects.

FIG. 4 provides a flowchart that illustrates a sequence of operationsthat may be performed by an example apparatus.

FIG. 5 provides a flowchart that illustrates a sequence of operationsthat may be performed by an example apparatus.

FIG. 6 provides a flowchart that illustrates a sequence of operationsthat may be performed by an example apparatus.

FIG. 7 provides a flowchart that illustrates a sequence of operationsthat may be performed by an example apparatus.

FIG. 8 provides a flowchart that illustrates a sequence of operationsthat may be performed by an example apparatus.

FIG. 9 provides a flowchart that illustrates a sequence of operationsthat may be performed by an example apparatus.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements. The figures are not necessarilyto scale, and the size of some parts may be exaggerated to more clearlyillustrate the example shown. Moreover the drawings provide examplesand/or implementations consistent with the description; however, thedescription is not limited to the examples and/or implementationsprovided in the drawings.

DESCRIPTION

Examples provided herein include apparatuses, processes, and methods forgenerating three-dimensional objects. Apparatuses for generatingthree-dimensional objects may be referred to as additive manufacturingapparatuses. As will be appreciated, example apparatuses describedherein may correspond to three-dimensional printing systems, which mayalso be referred to as three-dimensional printers. In an exampleadditive manufacturing process, a layer of build material may bedistributed in a build area of a build material support, an agent may beselectively distributed on the layer of build material, and energy maybe temporarily applied to the layer of build material. As used herein, abuild layer may refer to a layer of build material distributed in abuild area upon which agent may be distributed and/or energy may beapplied.

The application of energy may cause portions of the layer of buildmaterial to which agent has been applied to begin to coalesce or fuse.In contrast, build material upon which agent was not distributed may notcoalesce or fuse due to the application of energy. In some examples,energy may be applied to a build layer with at least one energy sourceof an example apparatus. Examples of energy sources may comprise aheating element, electromagnetic radiation emitter (e.g., light emitter,infrared emitter, ultraviolet emitter, laser emitter, etc.), and/or alamp (e.g., a halogen lamp).

The temperature at which portions of the layer of build material maybegin to coalesce or fuse may be referred to as a fusing temperature.Upon cooling, portions of the build material layer that have coalescedbecome solid and form part of a three-dimensional object beinggenerated. Accordingly, the portions of the build material layer uponwhich agent is selectively distributed (and which become solid) may bereferred to as the “part area” of the build layer.

Additional layers may be distributed and the operations described abovemay be performed for each layer to thereby generate a three-dimensionalobject. As will be appreciated, a part area of each layer may correspondto a cross-section of a three-dimensional object to be formed.Sequentially layering and fusing portions of layers of build material ontop of previous layers may facilitate generation of thethree-dimensional object. The layer-by-layer formation of athree-dimensional object may be referred to as a layer-wise additivemanufacturing process. In some examples, a height of a build layer(which may also be referred to as thickness) may be in the micron scale.For example, some example build layer heights may be in the range ofapproximately 60 to approximately 150 microns. Furthermore, an overallbuild height of the cumulative layers of build material may be in thecentimeter or meter scale depending on the apparatus and processimplemented.

In examples described herein, a build material may include apowder-based build material, where powder-based build material maycomprise wet and/or dry powder-based materials, particulate materials,and/or granular materials. In some examples, the build material may be aweak light absorbing polymer. In some examples, the build material maybe a thermoplastic. Furthermore, as described herein, agent may comprisefluids that may facilitate fusing of build material when energy isapplied. In some examples, agent may be referred to as coalescing orfusing agent. In some examples, agent may be a light absorbing fluid, aninfrared or near infrared absorbing fluid, such as a pigment colorant.

As will be appreciated, various types of build materials may havediffering material properties. Performance of various operations of alayer-wise additive manufacturing process as described herein may bebased at least in part on material properties of a build material usedin such process. Example material properties may include criticaltemperature (e.g., a temperature corresponding to a critical point thatis an end point of a phase equilibrium), emissivity, absorbance, thermalconductivity, thermal diffusivity, thermal expansion, photosensitivity,reflectivity, melting point, coefficient of thermal expansion,plasticity, elasticity, permeability, reactivity, surface energy,electrical conductivity, permittivity, and/or other such materialproperties.

In some examples, an apparatus may comprise a build material support,where a surface of the build material support may correspond to a buildarea. Accordingly, a starting layer of build material may be distributedon the surface of the build material support in the build area.Subsequent layers of build material may be distributed in the build areaon previously distributed and processed layers. Furthermore, apparatusesmay comprise energy sources with which to temporarily apply energy tobuild material layers to heat the build material layers to therebypre-fuse and/or fuse the build material layers. For example, anapparatus may comprise at least one energy source that may heat a buildmaterial layer such that portions of the build material layer whereagent has been distributed may fuse. In some examples, apparatuses mayfurther comprise additional energy sources with which to preheat buildmaterial layers.

In addition, example apparatus may comprise an agent distributor. Insome examples, an agent distributor may comprise a printhead orprintheads (e.g., a thermal ejection based printhead, a piezoelectricejection based printhead, etc.). In some examples, an agent distributormay be coupled to a scanning carriage. In one example, printheadssuitable for implementation in commercially available inkjet printingdevices may be implemented as an agent distributor. In other examples,an agent distributor may comprise other types of fluid ejection devicesthat selectively eject small volumes of fluid. In some examples, anagent distributor may comprise at least one printhead that comprises aplurality of fluid ejection dies arranged generally end-to-end along awidth of the agent distributor. In some examples, the at least oneprinthead may comprise a plurality of printheads arranged generallyend-to-end along a width of the agent distributor. In such examples, awidth of the agent distributor may correspond to a dimension of a buildarea. For example, a width of the agent distributor may correspond to awidth of a build area.

In some examples, apparatuses may comprise a build material distributorto distribute build material in the build area. A build materialdistributor may comprise, for example, a wiper blade, a roller, and/or aspray mechanism. In some examples, a build material distributor may becoupled to a scanning carriage. In these examples, the build materialdistributor may distribute build material in the build area as thescanning carriage moves over the build area along the scanning axis tothereby distribute a layer of build material in the build area.

In some examples of apparatuses for generating three-dimensionalobjects, build layer coverage corresponds to an amount of build materialdistributed at locations of the build area for a particular build layer.In particular, if build material is not evenly distributed for a buildlayer, build layer coverage may be insufficient for performingoperations of a layer-wise additive manufacturing process. As will beappreciated, build layer coverage may be related to overallthree-dimensional object generation accuracy and quality. For example,if build material was not distributed (which may be referred to asinsufficient build layer coverage) at some locations of a build layer,formation of a cross-section of an object corresponding to the buildlayer may not be accurate at such locations. As another example, if anamount of build material distributed at some locations of a build layerwas less than expected (which may be referred to as insufficient buildlayer coverage), formation of a cross-section of an object correspondingto the build layer may not be accurate at such locations. As usedherein, analyzing build layer coverage comprises determining whether asufficient or insufficient amount of build material has been distributedat locations of a build layer.

Example apparatuses, processes, and methods described herein analyzebuild layer coverage for a distributed build layer. In particular,examples may analyze build layer coverage based at least in part onbuild layer temperature, where build layer temperature includes measuredtemperatures at locations of a build layer. Hence, a build layertemperature generally corresponds to a temperature profile for a buildlayer that includes temperatures at various points (locations) of thebuild layer. In some examples, if a particular build layer isdistributed over a previous build layer in the build area, some examplesmay analyze build layer coverage of the particular build layer based atleast in part on a build layer temperature of the previous build layerand a build layer temperature of the particular build layer.

Example apparatuses comprise at least one build layer temperature sensorwith which to determine a build layer temperature of a layer of buildmaterial distributed in a build area of the apparatus. The build areamay be described with regard to a two-dimensional coordinate system suchthat locations of the build area may be defined according to atwo-dimensional coordinate system. Accordingly, locations of adistributed build layer may also be described with correspondinglocations. Examples herein may further determine a temperature for eachlocation of each build layer. Examples of a build layer temperaturesensor may comprise an infrared temperature sensor, a thermal visionsensor, infrared camera, and/or other types of sensors that may be usedto determine temperatures for locations of a build layer of buildmaterial.

As discussed above, a build layer is composed of build materialdistributed in the build area. For a given apparatus, a particular buildlayer height is expected for efficient and accurate operation inperforming operations of an additive manufacturing process. For example,some apparatuses operate with a build layer height in a range ofapproximately 60 to approximately 150 microns. In other examples, buildlayer heights may be in different ranges. Build layer coveragecorresponds to uniformity of build layer height at locations of a buildlayer and adequacy (e.g., sufficiency) of the amount of build materialat locations of the build layer. As will be appreciated, approximateuniformity of build material and within an expected range may be neededfor efficient and accurate operation of an example apparatus.Accordingly, examples may analyze build layer coverage for a build layerto determine whether the build layer uniformity is sufficient orinsufficient.

A sufficient build layer uniformity corresponds to a build layer havingan approximately uniform distribution of build material within anexpected range of build layer height. An insufficient build layeruniformity corresponds to a build layer that does not have anapproximately uniform distribution of build material or a build layerheight not within an expected range. As will be appreciated, build layerheight corresponds to an amount of build material distributed at arespective location. As used herein, “approximate” with regard tonumerical values may indicate a range of ±10%. As used with regard tobuild layer uniformity, approximately uniform indicates that buildmaterial height at locations of a build layer are within a range of±10%. Examples provided herein may analyze build layer coverage for abuild layer based at least in part on temperatures for locations of thebuild layer and temperatures of corresponding locations of a previouslayer.

Furthermore, in some examples described herein, a first build layer ofbuild material and a second build layer of build material may bedescribed. It will be appreciated that “first” and “second” are merelyused for illustrative purposes. For consistency, some examples providedherein describe a topmost build layer of build material as a secondbuild layer, and examples describe a build layer upon which a topmostlayer of build material is distributed as a first build layer.Accordingly, it will be appreciated that in some examples first andsecond build layers may be sequential layers in a layer-wise additivemanufacturing process. However, it will be noted that first and seconddo not necessarily describe an overall order of such build layers.

Turning now to the figures, and particularly to FIG. 1, this figureprovides a block diagram of some components of an example apparatus 10for generating a three-dimensional object. In this example, theapparatus 10 may comprise a build material support 12 having a surfacethat corresponds to a build area 14. In this example, the build materialsupport 12 is illustrated with dashed lines to reflect that a buildmaterial support may not be included in some examples. As will beappreciated, layers of build material may be distributed in the buildarea 14 on the surface of the build material support 12. Furthermore, inthis example, the apparatus 10 comprises a build material distributor 16to distribute respective build layers of build material in the buildarea 14. The apparatus 10 further comprises a controller 28 connected tothe at least one build material distributor 16. In such examples, thecontroller 28 is to control the build material distributor 16 todistribute a particular build layer in the build area 14 over a previousbuild layer. The controller 28 determines a build layer temperaturecorresponding to the particular build layer, and the controller analyzesbuild layer coverage for the particular build layer based at least inpart on a build layer temperature of the previous build layer and thebuild layer temperature of the particular build layer. As will beappreciated, the controller may perform the operations for each layer ofa layer-wise additive manufacturing process to generate athree-dimensional object. Furthermore, it will be appreciated, that insome examples, the controller 28 determines a build layer temperaturebased at least in part on sensor data received from one or moretemperature sensors.

A controller, such as the controller 28 of the example apparatus 10, maycomprise a processing resource and/or a memory resource. A processingresource may comprise one or more general purpose data processors and/orone or more specialized data processors. For example, a processingresource may comprise a central processing unit (CPU), a graphicsprocessing unit (GPU), an application-specific integrated circuit(ASIC), a controller, and/or other such configurations of logicalcomponents for data processing.

A memory resource may comprise random access memory (RAM) devices aswell as other types of memory (e.g. cache memories, non-volatile memorydevices, read-only memories, mass-storage resources, etc.). As will beappreciated, a memory resource may be a computer readable and/ormachine-readable storage medium (e.g., RAM, ROM, erasable programmableread-only memory (EPROM), electrically erasable programmable read-onlymemory, flash memory or other solid state memory technology, portablecompact disc memory, or other optical storage, or any other medium thatmay be used to store executable instructions and information).

Therefore, as described herein, a controller may be any combination ofhardware and programming to implement the functionalities described withrespect to a controller and/or a method. In some examples describedherein, the combinations of hardware and programming may be implementedin a number of different ways. For example, the programming for thecontroller may be processor executable instructions stored on anon-transitory machine-readable storage medium and the hardware for thecontroller may include a processing resource to execute thoseinstructions. In these examples, an apparatus implementing suchcontroller may include the machine-readable storage medium storing theinstructions and the processing resource to execute the instructions, orthe machine-readable storage medium may be separately stored andaccessible by the apparatus and the controller. In some examples, acontroller may be implemented in circuitry.

In the example apparatus 10 of FIG. 1, the controller 28 may control thebuild material distributor 16. In addition, the controller 28 may beconnected to a build layer temperature sensor (not shown in FIG. 1) andthe controller may receive sensor data from the build layer temperaturesensor. The sensor data may be processed by the controller 28 todetermine a build layer temperature of a build layer distributed in thebuild area 14. In some examples, the controller 28 may control operationof the build material distributor 16 based at least in part on buildlayer temperatures determined from the sensor data received from thebuild layer temperature sensor.

For example, the controller 28 may analyze build layer coverage for abuild layer in the build area 14, and the controller may control thebuild material distributor 16 to recoat the build layer with additionalbuild material in response to determining that the build layer coverageis insufficient. In some examples, if the controller 28 determinesparticular locations of the build layer having an insufficient amount ofbuild material, the controller may control the build material 16distributor such that additional build material is distributed at suchlocations. As used herein, recoating a build layer comprisesdistribution of an additional amount of build material for the buildlayer. The additional amount of build material may be less than anamount used for distributing a build layer. Moreover, in some examples,recoating a build layer comprises distribution of an additional amountof build material at locations of the build layer determined to haveinsufficient build layer coverage. In other examples, an additionalamount of build material may be distributed over the whole build areafor the build layer.

FIG. 2 provides a block diagram that illustrate some components of anexample apparatus 50. In this example, the apparatus 50 comprises abuild material support 52 having a surface that corresponds to a buildarea 54 in which layers of build material may be deposited and fused ina layer-wise additive manufacturing process to form a three-dimensionalobject. In this example, the apparatus 50 comprises a scanning carriage56 that may move bi-directionally along a scanning axis 58. In thisexample, the apparatus 50 includes a build material distributor 60, atleast one energy source 62, and an agent distributor 64 coupled to thescanning carriage 56. Accordingly, the build material distributor 60,the at least one energy source 62, and the agent distributor 64 may moveover the build area along the scanning axis 58 concurrent with movementof the scanning carriage 56.

As shown, a width of the build material distributor 60, at least oneenergy source 62, and the agent distributor may generally correspond toa width of the build area 54. Furthermore, the width of the buildmaterial distributor 60, the at least one energy source 62, and theagent distributor may be approximately orthogonal to the scanning axis58. Accordingly, as the scanning carriage 56 moves along the scanningaxis 58 over the build area 54, the build material distributor 60, theat least one energy source 62, and/or the agent distributor 64 mayperform operations associated with a layer-wise additive manufacturingprocess along a scan line that is approximately orthogonal to thescanning axis 58.

In addition, the apparatus 50 comprises at least one build layertemperature sensor 66. In such examples, a build layer temperature of abuild layer in the build area 54 may be determined based at least inpart on sensor data generated by the build layer temperature sensor 66.Therefore in this example, the apparatus 50 may coordinate operations tobe performed by the build material distributor 60, the at least oneenergy source 62, and/or the agent distributor 64 based at least in parton sensor data generated by the build layer temperature sensor 66.

Furthermore, the apparatus 50 comprises a pre-delivery receptacle 68that may store build material prior to distribution. In this example,build material may be stored in the pre-delivery receptacle 68, and thebuild material distributor 60 may remove build material from thepre-delivery receptacle 68 and distribute such build material in thebuild area 54. At least one heating element 70 (which may also bereferred to as a build material heating element) may be positionedproximate the pre-delivery receptacle 68, where the at least one heatingelement 70 may be used to heat build material stored in the pre-deliveryreceptacle 68 prior to distribution of the build material by the buildmaterial distributor 60. In some examples, the at least one heatingelement 70 may comprise an array of heating elements disposed in thepre-delivery receptacle 68. For example, the array of heating elementsmay comprise an array of resistive heating elements. In addition, theapparatus 50 comprises at least one pre-delivery receptacle temperaturesensor 72 that may be used to determine a temperature of build materialstored in the pre-delivery receptacle 68.

In this example, the apparatus 50 comprises a controller 74 that may beelectrically connected to the scanning carriage 56, the build materialdistributor 60, the energy source 62, the agent distributor 64, thebuild layer temperature sensor 66, the heating element 70, and/or thepre-delivery receptacle temperature sensor 72. As will be described infurther detail herein, the controller 74 may control the performance ofvarious operations of the scanning carriage 56, the build materialdistributor 60, the energy source 62, the agent distributor 64, thebuild layer temperature sensor 66, the heating element 70, and/or thepre-delivery receptacle temperature sensor 72.

FIG. 3 provides a block diagram that illustrates some components of anexample apparatus 100 for generating a three-dimensional object. In thisexample, the apparatus 100 may comprise a build material support 102. Insome examples, the build material support 102 may not be included in theapparatus 100; hence, the build material support 102 is illustrated indashed line. As discussed previously, the build material support 102 mayhave a build surface that corresponds to a build area 104 in whichlayers of build material may be sequentially distributed and fused.

The example apparatus 100 comprises a first scanning carriage 106 and asecond scanning carriage 108. In addition, the apparatus 100 includesbuild layer temperature sensors 110. In this example, the first scanningcarriage 106 may move bi-directionally over the build area 104 along afirst scanning axis 114, and the second scanning carriage 108 may movebi-directionally over the build area 104 along a second scanning axis116. As will be appreciated, the first scanning axis 114 isapproximately orthogonal to the second scanning axis 116. Furthermore,the first scanning axis 114 and the second scanning axis 116 areapproximately parallel to a plane of the surface of the build materialsupport 102.

The apparatus 100 comprises energy sources 118 and a build materialdistributor 120 coupled to the first scanning carriage 106. Accordingly,as the first scanning carriage 106 moves along the first scanning axis114, the apparatus 100 may distribute a layer of build material in thebuild area 104 with the build material distributor 120. Similarly, asthe first scanning carriage 106 moves bi-directionally along the firstscanning axis 114, the apparatus 100 may emit energy via the energysources 118 to apply energy to a build layer in the build area 104.

The apparatus 100 comprises an agent distributor 122 coupled to thesecond scanning carriage 108. Accordingly, as the second scanningcarriage moves along the second scanning axis 116, the apparatus mayselectively distribute agent on a build layer in the build area 104 withthe agent distributor 122. As will be appreciated, the examplearrangement of the agent distributor 122, energy sources 118, and/orbuild material distributor 120 may be different in other examples. Inanother example, a build material distributor and an agent distributormay be coupled to a common scanning carriage. Other examples maycomprise other arrangements.

As shown, the apparatus 100 further comprises a controller 130, wherethe controller is connected to the first scanning carriage 106, thesecond scanning carriage 108, the build layer temperature sensors 110,the energy sources 118, the build material distributor 120, and theagent distributor 122. Therefore, the controller 130 may receive sensordata from the build layer temperature sensors 110, and the controller130 may control operation of the first scanning carriage 106, the secondscanning carriage 108, the build layer temperature sensors 110, theenergy sources 118, the build material distributor 120, and the agentdistributor 122.

As shown, the controller 130 comprises at least one processing resource132 and a memory resource 134. As discussed previously, the processingresource 132 may comprise a data processor, CPU, ASIC, or other sucharrangements of logical components for processing data. Furthermore, thememory resource 134 may comprise a machine-readable storage medium. Thememory resource 136 comprises instructions 136, where the instructions136 are executable by the processing resource 132. Execution of theinstructions 136 may cause the processing resource 132 and/or apparatus100 to perform the functionalities, processes, and/or sequences ofoperations described herein.

For example, execution of the instructions 136 by the processingresource 132 may control movement of the scanning carriages 106, 108. Asanother example, execution of the instructions 136 may cause the buildlayer temperature sensors 110 to generate sensor data associated with abuild layer. In some examples, execution of the instructions 136 maycause the controller 130 to determine a first build layer temperaturefor a first build layer distributed in the build area with the buildlayer temperature sensors 110. In some examples, execution of theinstructions 136 may cause the controller 130 to control the buildmaterial distributor 120 to distribute a second build layer of buildmaterial on top of the first layer of build material. In addition,execution of the instructions may cause the controller 130 to determinea second build layer temperature for the second build layer with thebuild layer temperature sensors 110. Furthermore, execution of theinstructions may cause the controller 130 to analyze build layercoverage for the second build layer based at least in part on the secondbuild layer temperature and the first build layer temperature.

FIGS. 4-9 provide flowcharts that provide example sequences ofoperations that may be performed by an example apparatus and/or acontroller thereof to perform example processes and methods as describedherein. In some examples, some operations included in the flowcharts maybe embodied in a memory (such as the memory resource 134 of FIG. 3) inthe form of instructions that may be executable by a processing resourceto cause an apparatus and/or controller to perform the operationscorresponding to the instructions. Additionally, the examples providedin FIGS. 4-9 may be embodied in computing devices, machine-readablestorage mediums, processes, and/or methods. In some examples, theexample processes and/or methods disclosed in the flowcharts of FIGS.4-9 may be performed by a controller implemented in an apparatus.

FIG. 4 is a flowchart 150 that illustrates an example sequence ofoperations that may be performed by an example apparatus for generatinga three-dimensional object. In this example, a controller may control abuild material distributor of the apparatus to distribute a particularbuild layer in a build area of the apparatus over a previous build layer(block 152). The controller may determine a build layer temperature forthe particular build layer with a build layer temperature sensor of theapparatus (block 154). The controller may analyze build layer coveragefor the particular build layer based at least in part on the build layertemperature for the particular build layer and a build layer temperatureof the previous build layer (block 156). Based at least in part on thebuild layer coverage determined for the particular build layer, thecontroller may control the build material distributor to selectivelyrecoat the particular build material with additional build material(block 158).

As will be appreciated, controlling the build material distributor toselectively recoat the particular build layer may comprise: controllingthe build material distributor to recoat the particular build layer withadditional build material if the build layer coverage of the particularbuild layer is insufficient; and not controlling the build materialdistributor to recoat the particular build layer with additional buildmaterial if the build layer coverage of the particular build layer issufficient. As will be appreciated, if the build layer coverage of theparticular build layer is sufficient, the controller may proceed toperforming other operations associated with a layer-wise additivemanufacturing process (e.g., controlling an agent distributor todistribute agent on the particular layer, controlling an energy sourceto apply energy to the particular build layer, etc.).

FIG. 5 is a flowchart 200 that illustrates an example sequence ofoperations that may be performed by an example apparatus for generatinga three-dimensional object. In this example, a controller of theapparatus may determine a build layer temperature for a first buildlayer with a build layer temperature sensor of the apparatus (block202). Furthermore, in this example, the controller may determine apre-delivery build material temperature for build material stored in apre-delivery receptacle with a pre-delivery receptacle temperaturesensor (block 204).

A heating element of the apparatus that is positioned proximate thepre-delivery receptacle to pre-heat build material prior to distributionmay be controlled by the controller based at least in part on thepre-delivery build material temperature and the build layer temperatureof the first build layer (block 206). In some examples, the controllermay pre-heat build material before distributing build material, wherepre-heating of the build material may facilitate improved objectformation accuracy as well as reduce energy usage in fusing operations.In these examples, build material may be pre-heated to an optimalworking temperature, where the optimal working temperature may be basedat least in part on material properties of the build material. Inaddition, the optimal working temperature may be based at least in parton a build layer temperature of a build layer upon which the buildmaterial is to be distributed. For example, build material to bedistributed may be pre-heated to a temperature that is within apredefined range of the build layer temperature upon which the buildmaterial is to be distributed. In some examples, build material may bepreheated to a range of approximately 150° Celsius to approximately 185°Celsius. As will be appreciated, preheating of build material may bebased at least in part on material properties of the build material. Forexample, a build material composed of Polyamide 11 (PA-11) (also knownas Nylon 11) may be preheated to a range of approximately 155° Celsiusto approximately 165° Celsius. As another example, a build materialcomposed of Polyamide 12 (PA-12) (also known as Nylon 12) may bepreheated to a range of approximately 165° Celsius to approximately 175°Celsius.

The controller may control a build material distributor of the apparatusto distribute pre-heated build material in the build area on top of thefirst build layer as a second build layer (block 208). Afterdistributing build material for the second build layer, the controllermay determine a build layer temperature for the second build layer(block 210). The controller may analyze build layer coverage for thesecond build layer based at least in part on the build layer temperaturefor the first build layer and the build layer temperature for the secondbuild layer (block 212).

As discussed previously, a build layer temperature includes atemperature for each location of a build layer. As will be appreciated,temperatures at locations of a build layer may vary. In some examples,analyzing a build layer coverage may comprise identifying locations of abuild layer at which a temperature is greater than the temperature ofproximate locations of the build layer. In such examples, locations atwhich the temperature is greater than proximate locations may correspondto an area where a sufficient amount of build material was notdistributed.

FIG. 6 provides a flowchart 250 that illustrates an example sequence ofoperations that may be performed by an example apparatus. In thisexample, a controller of the example apparatus may determine a buildlayer temperature of a first build layer with a build layer temperaturesensor (block 252). The controller controls a build material distributorof the apparatus to distribute a second build layer of build material inthe build area and on top of the first build layer (block 254). Thecontroller may determine a build layer temperature of the second buildlayer with the build layer temperature sensor (block 254). Thecontroller analyzes build layer coverage for the second build layerbased at least in part on the build layer temperature of the first buildlayer and the build layer temperature of the second build layer todetermine whether build layer coverage is sufficient or insufficient(blocks 258-260).

In response to determining that the build layer coverage for the secondbuild layer is insufficient (i.e., not sufficient) (“N” branch of block260), the controller may control the build material distributor torecoat the second build layer with additional build material (block262). For example, if a group of neighboring locations (e.g., an area)of the second build layer is determined to have an inadequate amount ofbuild material, the example apparatus may determine that build layercoverage is insufficient. In this example, the controller may controlthe build material distributor to distribute additional build materialat the group of neighboring locations of the second build layer. Afterrecoating the second build layer, the controller may repeat determiningthe build layer temperature for the second build layer and analyzing thebuild layer coverage (blocks 256-260).

In response to determining that the build layer coverage for the secondbuild layer is sufficient (“Y” branch of block 260), the controller maycontrol an agent distributor of the apparatus to selectively distributeagent in the build area on the second build area (block 264). Afterselectively distributing agent, the controller may control at least oneenergy source of the apparatus to temporarily apply energy to the secondbuild layer (block 266). Accordingly, examples described herein maycontrol operation of various components of an example apparatus based atleast in part on build layer coverage of a build layer. In turn, buildlayer coverage may be analyzed based at least in part on a build layertemperature of a topmost build layer (e.g., a second build layer) and abuild layer temperature of a build layer upon which the topmost buildlayer was distributed (e.g., a first build layer).

FIG. 7 provides a flowchart 300 that illustrates an example sequence ofoperations that may be performed by an apparatus to analyze build layercoverage of a build layer during generation of a three-dimensionalobject. In this example, for a first build layer temperature of a firstbuild layer and a second build layer temperature of a second build layer(block 302), the apparatus (and a controller thereof) may comparetemperatures of corresponding locations for the first build layer andthe second build layer (block 304). As will be appreciated, the secondbuild layer corresponds to a topmost build layer upon which variousoperations of a layer-wise additive manufacturing process are beingperformed, and the first build layer corresponds to a build layer uponwhich the second build layer is distributed. Hence, operations of alayer-wise additive manufacturing process have been performed on thefirst build layer. It will be appreciated that in some examples fusinghas been performed for the first build layer prior to distribution ofthe second build layer. Therefore, in some examples, the build layertemperature of the first build layer may be greater than the build layertemperature for the second build layer after distribution of the secondbuild layer.

Moreover, as discussed previously, a location of a build layer may bedescribed relative to a position on the build area, where correspondinglocations for different build layers vary in the build direction (i.e.,vertically). Therefore, examples similar to the example of FIG. 7 mayanalyze a temperature for a location of the first build layer and atemperature for a corresponding location of the second build layer.

Examples detect temperature edges for locations of the second build area(block 306). A temperature edge may correspond to a group of neighboringlocations (i.e., an area) at which a temperature is at least 5° Celsiusdifferent than temperatures for proximate locations. Furthermore, atemperature edge may correspond to a group of neighboring locations fora second build layer for which temperatures thereof are within apredefined range of temperatures of corresponding locations for a firstbuild layer. Detection of temperature edges for locations of a buildlayer may correspond to locations at which an inadequate amount of buildmaterial was distributed for the build layer.

For example, temperatures for locations of a second build layer may beapproximately equal to temperatures for corresponding locations of afirst build layer if no build material was distributed for the secondbuild layer at the locations (which may be referred to as a hole). Asanother example, temperatures for locations of a second build layer maybe within a predefined range (e.g., 5° Celsius, 10° Celsius, etc.) oftemperatures for corresponding locations of a first build layer if aninadequate/insufficient amount of build material was distributed at thelocations of the second build layer (which may be referred to as alightly covered area).

Therefore, based on the detection of temperature edges for temperaturesof locations of the second build layer, the apparatus may determinewhether any holes or lightly covered areas are present for the secondbuild layer (block 308). In response to determining that holes orlightly covered areas are not present (“N” branch of block 308), theapparatus may determine that the build layer coverage is sufficient forthe second build layer (block 310). In response to determining thepresence of holes or lightly covered areas for the second build layer(“Y” branch of block 308), the apparatus may determine whether holes orlight coverage areas present in the second build layer correspond to thethree-dimensional object to be generated (block 312). As will beappreciated, in some examples, if a hole or light coverage is present ina build layer, but the locations where such hole or light coverageoccurs does not correspond to the part area or the object to begenerated (“N” branch of block 312), some examples may determine thatthe build layer coverage is sufficient (block 310). However, in responseto determining that the hole or light coverage area corresponds to thepart area or object to be generated (“Y” branch of block 312), someexamples may determine that the build layer coverage is insufficient(block 314).

Turning now to FIG. 8, this figure provides a flowchart 350 thatillustrates an example sequence of operations that may be performed byan example apparatus to generate a three-dimensional object. As shown,the apparatus may apply energy to a first build layer of build materialin the build area with an energy source (block 352). As will beappreciated, the application of energy may cause fusing of a portion ofthe first build layer that corresponds to the three-dimensional objectto be generated. The apparatus may distribute a second build layer ofbuild material over the first layer of build material in the build areawith a build material distributor (block 354). The apparatus maydetermine a build layer temperature for the second build layer with abuild layer temperature sensor (block 356). The apparatus may analyzebuild layer coverage of the second build layer based at least in part onthe build layer temperature for the second build layer and a build layertemperature for the first build layer (block 358). Based at least inpart on the build layer coverage for the locations of the second buildlayer, examples may selectively recoat the second build layer withadditional build material (block 360). In some examples, selectivelyrecoating the second build layer may comprise recoating the second buildlayer if build layer coverage is insufficient, and selectively recoatingthe second build layer may comprise not recoating the second build layerif build layer coverage is sufficient.

FIG. 9 provides a flowchart 400 that illustrates an example sequence ofoperations that may be performed by an example apparatus to generate athree-dimensional object. As shown, the apparatus may apply energy to afirst build layer of build material in a build area of the apparatuswith an energy source (block 402). The apparatus may determine a firstbuild layer temperature for the first build layer with a build layertemperature sensor (block 404). The apparatus may determine apre-delivery build material temperature for build material to bedistributed in the build area with a pre-delivery receptacle temperaturesensor (block 406).

Based on the pre-delivery build material temperature and the first buildlayer temperature, the apparatus may pre-heat the build material to bedistributed in the build area with at least one heating element (block408). As will be appreciated, the build material may be pre-heated to apredefined optimal temperature for performing layer-wise additivemanufacturing operations thereon. After pre-heating the build material,the apparatus may distribute the pre-heated build material in the buildarea on top of the first build layer as a second build layer of buildmaterial with a build material distributor (block 410).

The apparatus may determine a second build layer temperature for thesecond build layer with the build layer temperature sensor (block 412).Based at least in part on the first build layer temperature and thesecond build layer temperature, the apparatus may analyze build layercoverage for the second build layer to determine whether build layercoverage for the second build layer is sufficient (blocks 414-416). Inresponse to determining that build layer coverage is insufficient (“N”branch of block 416), the apparatus may recoat the second build layerwith additional build material with the build material distributor(block 418). In response to determining that the build layer coveragefor the second build layer is sufficient (“Y” branch of block 416), theapparatus may selectively distribute agent on the second build layer inthe build area with an agent distributor (block 420).

Therefore, example apparatuses and processes described herein facilitatebuild layer coverage analysis based at least in part on build layertemperatures. Furthermore, examples may pre-heat build material prior todistribution thereof based at least in part on a build layer temperaturefor a build layer upon which the build material is to be distributed. Aswill be appreciated, some examples may compensate for insufficient buildlayer coverage by recoating a build layer with additional build materialin locations in which build material coverage is inadequate. Examplesdescribed herein may facilitate improved selectivity in apparatuses andmethods for generating three-dimensional objects. Furthermore, examplesmay improve mechanical strength for objects generated with apparatuses,methods, and processes described herein.

The preceding description has been presented to illustrate and describeexamples of the principles described. This description is not intendedto be exhaustive or to limit these principles to any precise formdisclosed. Many modifications and variations are possible in light ofthe above disclosure.

1. An apparatus for generating a three-dimensional object, the apparatuscomprising: a build material distributor to distribute respective buildlayers of build material in a build area of the apparatus; and acontroller to: control the build material distributor to distribute aparticular build layer in the build area over a previous build layer;determine a build layer temperature corresponding to the particularbuild layer; analyze build layer coverage for the particular build layerbased at least in part on a build layer temperature of the previousbuild layer and the build layer temperature of the particular buildlayer; and control the build material distributor to selectively recoatthe particular build layer with additional build material based at leastin part on the build layer coverage for the particular build layer. 2.The apparatus of claim 1, further comprising: at least one build layertemperature sensor, wherein the controller determines the build layertemperature of the particular build layer with the at least onetemperature sensor.
 3. The apparatus of claim 1, wherein the controllerto control the build material distributor to selectively recoat theparticular build layer with additional build material comprises thecontroller to control the build material distributor to recoat theparticular build layer with additional build material in response todetermining that the build layer coverage for the particular build layeris insufficient.
 4. The apparatus of claim 1, further comprising: anagent distributor to selectively distribute agent in the build area; andat least one energy source, wherein the controller is further to:control the agent distributor to selectively distribute agent on theparticular build layer in response to determining that build layercoverage for the particular layer is sufficient; and control the atleast one energy source to apply energy to the particular build layerafter selective distribution of the agent on the particular build layer.5. The apparatus of claim 1, wherein the controller to analyze buildlayer coverage for the particular build layer comprises the controllerto: detect temperature edges for locations of the particular build layerbased at least in part on the build layer temperature for the previousbuild layer.
 6. The apparatus of claim 1, further comprising: at leastone build material heating element to heat build material prior todistribution in the build area by the build material distributor,wherein the controller is further to: prior to controlling the buildmaterial distributor to distribute the particular build layer, controlthe at least one build material heating element to thereby heat buildmaterial to be distributed as the particular build layer based at leastin part on the build layer temperature of the previous build layer. 7.The apparatus of claim 6, wherein the build material distributorcomprises at least one pre-delivery receptacle to store build materialprior to distribution, the at least one build material heating elementis disposed within the pre-delivery receptacle to heat build materialstored therein, wherein the apparatus further comprises at least onepre-delivery receptacle temperature sensor, wherein the controller isfurther to: determine a pre-delivery build material temperature with theat least one pre-delivery receptacle temperature sensor, wherein thecontroller controls the at least one build material heating element tothereby heat the build material to be distributed based at least in parton the pre-delivery build material temperature.
 8. A method for anapparatus for generating a three-dimensional object, the methodcomprising: applying energy, with at least one energy source of theapparatus, to a first build layer of build material distributed in abuild area of the apparatus; distributing, with a build materialdistributor of the apparatus, a second build layer of build material inthe build area over the first build layer; determining, with at leastone build layer temperature sensor, a build layer temperaturecorresponding to the second build layer; analyzing build layer coveragefor locations of the second build layer based at least in part oncomparing a build layer temperature of the first build layer and thebuild layer temperature of the second build layer, and selectivelyrecoating, with the build material distributor, the second build layerwith additional build material based at least in part on the build layercoverage for the locations of the second build layer.
 9. The method ofclaim 8, further comprising: prior to distributing the second buildlayer of build material, pre-heating the build material to bedistributed for the second build layer with at least one build materialheating element based at least in part on the build layer temperature ofthe first build layer.
 10. The method of claim 8, wherein analyzingbuild layer coverage for the locations of the second build layercomprises determining whether build layer coverage for the second buildlayer is sufficient or insufficient based at least in part on the buildlayer temperature for the first build layer and the build layertemperature for the second build layer, wherein selectively recoatingthe second build layer with additional build material comprises: inresponse to determining that the build layer coverage for the secondbuild layer is insufficient, recoating the second build layer withadditional build material with the build material distributor; and inresponse to determining that the build layer coverage for the secondlayer is sufficient, not recoating the second build layer withadditional build material and selectively distributing agent on thesecond build layer with an agent distributor of the apparatus.
 11. Themethod of claim 8, wherein analyzing build layer coverage for the secondbuild layer comprises detecting temperature edges for the locations ofthe second build layer based at least in part on the build layertemperature of the first build layer.
 12. An apparatus for generating athree-dimensional object, the apparatus comprising: a build materialdistributor to distribute respective build layers of build material in abuild area of the apparatus; at least one build layer temperaturesensor; at least one build material heating element to heat buildmaterial to be distributed by the build material distributor; and acontroller to: determine a first build layer temperature thatcorresponds to a first build layer of build material distributed in thebuild area; control the at least one build material heating element topre-heat build material to be distributed on the first build layer basedat least in part on the first build layer temperature; and afterpre-heating the build material to be distributed on the first buildlayer, control the build material distributor to distribute thepre-heated build material as a second build layer on the first layer ofbuild material.
 13. The apparatus of claim 12, wherein the controller isfurther to: determine a second build layer temperature that correspondsto the second build layer; and determine build layer coverage for thesecond build layer based at least in part on the first build layertemperature and the second build layer temperature.
 14. The apparatus ofclaim 13, further comprising: at least one energy source; and an agentdistributor to selectively distribute agent in the build area, whereinthe controller is further to: in response to determining that buildlayer coverage for the second build layer is sufficient, control theagent distributor to selectively distribute agent on the second buildlayer, and control the at least one energy source to apply energy to thesecond build layer, and in response to determining that build layercoverage for the second build layer is insufficient, control the buildmaterial distributor to recoat the second build layer with additionalbuild material, control the agent distributor to selectively distributeagent on the second build layer, and control the at least one energysource to apply energy to the second build layer.
 15. The apparatus ofclaim 13, wherein the build material distributor comprises apre-delivery receptacle to store build material prior to distribution,the at least one build material heating element is disposed proximatethe pre-delivery receptacle, wherein the apparatus further comprises atleast one pre-delivery receptacle temperature sensor, wherein thecontroller is further to: determine a pre-delivery build materialtemperature with the at least one pre-delivery receptacle temperaturesensor, wherein the controller controls the at least one build materialheating element to thereby heat the build material to be distributed onthe first build layer based at least in part on the pre-delivery buildmaterial temperature.